Accessibility Overview

Accessibility means ensuring that all users, including users with disabilities, have equal access to software. One piece of this involves basic design principles such as using appropriate font sizes and color contrast, avoiding using color to convey important information, and providing keyboard alternatives for anything that is normally accomplished with a pointing device. However, when you see the word “accessibility” in a directory name in Chromium, that code‘s purpose is to provide full access to Chromium’s UI via external accessibility APIs that are utilized by assistive technology.

Assistive technology here refers to software or hardware which makes use of these APIs to create an alternative interface for the user to accommodate some specific needs, for example:

Assistive technology includes:

Screen readers for blind users that describe the screen using synthesized speech or braille

Voice control applications that let you speak to the computer,

Switch access that lets you control the computer with a small number of physical switches,

Magnifiers that magnify a portion of the screen, and often highlight the cursor and caret for easier viewing, and

Assistive learning and literacy software that helps users who have a hard time reading print, by highlighting and/or speaking selected text

In addition, because accessibility APIs provide a convenient and universal way to explore and control applications, they're often used for automated testing scripts, and UI automation software like password managers.

Web browsers play an important role in this ecosystem because they need to not only provide access to their own UI, but also provide access to all of the content of the web.

Each operating system has its own native accessibility API. While the core APIs tend to be well-documented, it's unfortunately common for screen readers in particular to depend on additional undocumented or vendor-specific APIs in order to fully function, especially with web browsers, because the standard APIs are insufficient to handle the complexity of the web.

Chromium needs to support all of these operating system and vendor-specific accessibility APIs in order to be usable with the full ecosystem of assistive technology on all platforms. Just like Chromium sometimes mimics the quirks and bugs of older browsers, Chromium often needs to mimic the quirks and bugs of other browsers' implementation of accessibility APIs, too.

Concepts

While each operating system and vendor accessibility API is different, there are some concepts all of them share.

The tree, which models the entire interface as a tree of objects, exposed to assistive technology via accessibility APIs;

Events, which let assistive technology know that a part of the tree has changed somehow;

Actions, which come from assistive technology and ask the interface to change.

Note that the tree structure closely resembles the structure of the HTML elements, but slightly simplified. Each node in the accessibility tree has an ID and a role. Many have a name. The text field has a value, and instead of a name it has labelledByIds, which indicates that its accessible name comes from another node in the tree, the label node with id=2.

On a particular platform, each node in the accessibility tree is implemented by an object that conforms to a particular protocol.

On Windows, the root node implements the IAccessible protocol and if you call IAccessible::get_accRole, it returns ROLE_SYSTEM_DOCUMENT, and if you call IAccessible::get_accName, it returns “How old are you?”. Other methods let you walk the tree.

On macOS, the root node implements the NSAccessibility protocol and if you call [NSAccessibility accessibilityRole], it returns @“AXWebArea”, and if you call [NSAccessibility accessibilityLabel], it returns “How old are you?”.

The Linux accessibility API, ATK, is more similar to the Windows APIs; they were developed together. (Chrome's support for desktop Linux accessibility is unfinished.)

The Android accessibility API is of course based on Java. The main data structure is AccessibilityNodeInfo. It doesn't have a role, but if you call AccessibilityNodeInfo.getClassName() on the root node it returns “android.webkit.WebView”, and if you call AccessibilityNodeInfo.getContentDescription() it returns “How old are you?”.

On Chrome OS, we use our own accessibility API that closely maps to Chrome's internal accessibility API.

So while the details of the interface vary, the underlying concepts are similar. Both IAccessible and NSAccessibility have a concept of a role, but IAccessible uses a role of “document” for a web page, while NSAccessibility uses a role of “web area”. Both IAccessible and NSAccessibility have a concept of the primary accessible text for a node, but IAccessible calls it the “name” while NSAccessibility calls it the “label”, and Android calls it a “content description”.

Historical note: The internal names of roles and attributes in Chrome often tend to most closely match the macOS accessibility API because Chromium was originally based on WebKit, where most of the accessibility code was written by Apple. Over time we're slowly migrating internal names to match what those roles and attributes are called in web accessibility standards, like ARIA.

Accessibility Events

In Chromium's internal terminology, an Accessibility Event always represents communication from the app to the assistive technology, indicating that the accessibility tree changed in some way.

As an example, if the user were to press the Tab key and the text field from the example above became focused, Chromium would fire a “focus” accessibility event that assistive technology could listen to. A screen reader might then announce the name and current value of the text field. A magnifier might zoom the screen to its bounding box. If the user types some text into the text field, Chromium would fire a “value changed” accessibility event.

As with nodes in the accessibility tree, each platform has a slightly different API for accessibility events. On Windows we‘d fire EVENT_OBJECT_FOCUS for a focus change, and on Mac we’d fire @“AXFocusedUIElementChanged”. Those are pretty similar. Sometimes they're quite different - to support live regions (notifications that certain key parts of a web page have changed), on Mac we simply fire @“AXLiveRegionChanged”, but on Windows we need to fire IA2_EVENT_TEXT_INSERTED and IA2_EVENT_TEXT_REMOVED events individually on each affected node within the changed region, with additional attributes like “container-live:polite” to indicate that the affected node was part of a live region. This discussion is not meant to explain all of the technical details but just to illustrate that the concepts are similar, but the details of notifying software on each platform about changes can vary quite a bit.

Accessibility Actions

Each native object that implements a platform's native accessibility API supports a number of actions, which are requests from the assistive technology to control or change the UI. This is the opposite of events, which are messages from Chromium to the assistive technology.

For example, if the user had a voice control application running, such as Voice Access on Android, the user could just speak the name of one of the buttons on the page, like “Next”. Upon recognizing that text and finding that it matches one of the UI elements on the page, the voice control app executes the action to click the button id=6 in Chromium's accessibility tree. Internally we call that action “do default” rather than click, since it represents the default action for any type of control.

Other examples of actions include setting focus, changing the value of a control, and scrolling the page.

Parameterized attributes

In addition to accessibility attributes, events, and actions, native accessibility APIs often have so-called “parameterized attributes”. The most common example of this is for text - for example there may be a function to retrieve the bounding box for a range of text, or a function to retrieve the text properties (font family, font size, weight, etc.) at a specific character position.

Parameterized attributes are particularly tricky to implement because of Chromium's multi-process architecture. More on this below.

Tools for inspecting the Accessibility tree

Developers can inspect the accessibility tree in several ways:

By navigating to chrome://accessibility/ and inspecting a tree directly. Note that you may want to enable the ‘Internal’ option. Click ‘show accessibility tree’ for a particular tab, then click again to refresh that tree.

Chromium's multi-process architecture

Native accessibility APIs tend to have a functional interface, where Chromium implements an interface for a canonical accessible object that includes methods to return various attributes, walk the tree, or perform an action like click(), focus(), or setValue(...).

In contrast, the web has a largely declarative interface. The shape of the accessibility tree is determined by the DOM tree (occasionally influenced by CSS), and the accessible semantics of a DOM element can be modified by adding ARIA attributes.

One important complication is that all of these native accessibility APIs are synchronous, while Chromium is multi-process, with the contents of each web page living in a different process than the process that implements Chromium‘s UI and the native accessibility APIs. Furthermore, the renderer processes are sandboxed, so they can’t implement operating system APIs directly.

Chromium‘s multi-process architecture means that we can’t implement accessibility APIs the same way that a single-process browser can - namely, by calling directly into the DOM to compute the result of each API call. For example, on some operating systems there might be an API to get the bounding box for a particular range of characters on the page. In other browsers, this might be implemented by creating a DOM selection object and asking for its bounding box.

That implementation would be impossible in Chromium because it‘d require blocking the main thread while waiting for a response from the renderer process that implements that web page’s DOM. (Not only is blocking the main thread strictly disallowed, but the latency of doing this for every API call makes it prohibitively slow anyway.) Instead, Chromium takes an approach where a representation of the entire accessibility tree is cached in the main process. Great care needs to be taken to ensure that this representation is as concise as possible.

In Chromium, we build a data structure representing all of the information for a web page's accessibility tree, send the data structure from the renderer process to the main browser process, cache it in the main browser process, and implement native accessibility APIs using solely the information in that cache.

As the accessibility tree changes, tree updates and accessibility events get sent from the renderer process to the browser process. The browser cache is updated atomically in the main thread, so whenever an external client (like assistive technology) calls an accessibility API function, we‘re always returning something from a complete and consistent snapshot of the accessibility tree. From time to time, the cache may lag what’s in the renderer process by a fraction of a second.

Here are some of the specific challenges faced by this approach and how we've addressed them.

Sparse data

There are a lot of possible accessibility attributes for any given node in an accessibility tree. For example, there are more than 150 unique accessibility API methods that Chrome implements on the Windows platform alone. We need to implement all of those APIs, many of which request rather rare or obscure attributes, but storing all possible attribute values in a single struct would be quite wasteful.

To avoid each accessible node object containing hundreds of fields the data for each accessibility node is stored in a relatively compact data structure, ui::AXNodeData. Every AXNodeData has an integer ID, a role enum, and a couple of other mandatory fields, but everything else is stored in attribute arrays, one for each major data type.

So if a text field has a placeholder attribute, we can store that by adding an entry to string_attributes with an attribute of ax::mojom::StringAttribute::kPlaceholder and the placeholder string as the value.

Incremental tree updates

Web pages change frequently. It'd be terribly inefficient to send a new copy of the accessibility tree every time any part of it changes. However, the accessibility tree can change shape in complicated ways - for example, whole subtrees can be reparented dynamically.

Rather than writing code to deal with every possible way the accessibility tree could be modified, Chromium has a general-purpose tree serializer class that's designed to send small incremental updates of a tree from one process to another. The tree serializer has just a few requirements:

Every node in the tree must have a unique integer ID.

The tree must be acyclic.

The tree serializer must be notified when a node's data changes.

The tree serializer must be notified when the list of child IDs of a node changes.

The tree serializer doesn‘t know anything about accessibility attributes. It keeps track of the previous state of the tree, and every time the tree structure changes (based on notifications of a node changing or a node’s children changing), it walks the tree and builds up an incremental tree update that serializes as few nodes as possible.

In the other process, the Unserialization code applies the incremental tree update atomically.

Text bounding boxes

One challenge faced by Chromium is that accessibility clients want to be able to query the bounding box of an arbitrary range of text - not necessarily just the current cursor position or selection. As discussed above, it‘s not possible to block Chromium’s main browser process while waiting for this information from Blink, so instead we cache enough information to satisfy these queries in the accessibility tree.

To compactly store the bounding box of every character on the page, we split the text into inline text boxes, sometimes called text runs. For example, in a typical paragraph, each line of text would be its own inline text box. In general, an inline text box or text run contians a sequence of text characters that are all oriented in the same direction, in a line, with the same font, size, and style.

Each inline text box stores its own bounding box, and then the relative x-coordinate of each character in its text (assuming left-to-right). From that it's possible to compute the bounding box of any individual character.

For example, suppose that a document contains a text field with the text “Hello world”, but the field is narrow, so “Hello” is on the first line and “World” is on the second line. Internally Chromium's accessibility tree might look like this:

Scrolling, transformations, and animation

Native accessibility APIs typically want the bounding box of every element in the tree, either in window coordinates or global screen coordinates. If we stored the global screen coordinates for every node, we'd be constantly re-serializing the whole tree every time the user scrolls or drags the window.

Instead, we store the bounding box of each node in the accessibility tree relative to its offset container, which can be any ancestor. If no offset container is specified, it's assumed to be the root of the tree.

In addition, any offset container can contain scroll offsets, which can be used to scroll the bounding boxes of anything in that subtree.

Finally, any offset container can also include an arbitrary 4x4 transformation matrix, which can be used to represent arbitrary 3-D rotations, translations, and scaling, and more. The transformation matrix applies to the whole subtree.

Storing coordinates this way means that any time an object scrolls, moves, or animates its position and scale, only the root of the scrolling or animation needs to post updates to the accessibility tree. Everything in the subtree remains valid relative to that offset container.

Computing the global screen coordinates for an object in the accessibility tree just means walking up its ancestor chain and applying offsets and occasionally multiplying by a 4x4 matrix.

Site isolation / out-of-process iframes

At one point in time, all of the content of a single Tab or other web view was contained in the same Blink process, and it was possible to serialize the accessibility tree for a whole frame tree in a single pass.

Today the situation is a bit more complicated, as Chromium supports out-of-process iframes. (It also supports “browser plugins” such as the <webview> tag in Chrome packaged apps, which embeds a whole browser inside a browser, but for the purposes of accessibility this is handled the same as frames.)

Rather than a mix of in-process and out-of-process frames that are handled differently, Chromium builds a separate independent accessibility tree for each frame. Each frame gets its own tree ID, and it keeps track of the tree ID of its parent frame (if any) and any child frames.

In Chrome's main browser process, the accessibility trees for each frame are cached separately, and when an accessibility client (assistive technology) walks the accessibility tree, Chromium dynamically composes all of the frames into a single virtual accessibility tree on the fly, using those aforementioned tree IDs.

The node IDs for accessibility trees only need to be unique within a single frame. Where necessary, separate unique IDs are used within Chrome‘s main browser process. In Chromium accessibility, a “node ID” always means that ID that’s only unique within a frame, and a “unique ID” means an ID that's globally unique.

Blink

Blink constructs an accessibility tree (a hierarchy of WebAXObjects) from the page it is rendering. WebAXObject is the public API wrapper around [AXObject], which is the core class of Blink's accessibility tree. AXObject is an abstract class; the most commonly used concrete subclass of it is AXNodeObject, which wraps a Node. In turn, most AXNodeObjects are actually AXLayoutObjects, which wrap both a Node and a LayoutObject. Access to the LayoutObject is important because some elements are only in the AXObject tree depending on their visibility, geometry, linewrapping, and so on. There are some subclasses of AXLayoutObject that implement special-case logic for specific types of Node. There are also other subclasses of AXObject, which are mostly used for testing.

Note that not all AXLayoutObjects correspond to actual Nodes; some are synthetic layout objects which group related inline elements or similar.

The central class responsible for dealing with accessibility events in Blink is AXObjectCacheImpl, which is responsible for caching the corresponding AXObjects for Nodes or LayoutObjects. This class has many methods named handleFoo, which are called throughout Blink to notify the AXObjectCacheImpl that it may need to update its tree. Since this class is already aware of all accessibility events in Blink, it is also responsible for relaying accessibility events from Blink to the embedding content layer.

The content layer

The content layer lives on both sides of the renderer/browser split. The content layer translates WebAXObjects into AXContentNodeData, which is a subclass of ui::AXNodeData. The ui::AXNodeData class and related classes are Chromium's cross-platform accessibility tree. The translation is implemented in BlinkAXTreeSource. This translation happens on the renderer side, so the ui::AXNodeData tree now needs to be sent to the browser, which is done by sending AccessibilityHostMsg_EventParams with the payload being serialized delta-updates to the tree, so that changes that happen on the renderer side can be reflected on the browser side.

Merging AXNodeData trees into one tree of BrowserAccessibility objects, by linking to other BrowserAccessibilityManagers. This is important because each page has its own accessibility tree, but each Chromium window must have only one accessibility tree, so trees from multiple pages need to be combined (possibly also with trees from Views UI).

Dispatching outgoing accessibility events to the platform's accessibility APIs. This is done in the platform-specific subclasses of BrowserAccessibilityManager, in a method named NotifyAccessibilityEvent.

On Chrome OS, RenderFrameHostImpl does not route events to BrowserAccessibilityManager at all, since there is no platform screenreader outside Chromium to integrate with.

Views

Views generates a NativeViewAccessibility for each View, which is used as the delegate for an AXPlatformNode representing that View. This part is relatively straightforward, but then the generated tree must be combined with the web accessibility tree, which is handled by BrowserAccessibilityManager.

WebUI

Since WebUI surfaces have renderer processes as normal, WebUI accessibility goes through the blink-to-content-to-platform pipeline described above. Accessibility for WebUI is largely implemented in JavaScript in webui-js; these classes take care of adding ARIA attributes and so on to DOM nodes as needed.